Communications systems employing optical fiber cables are proliferating, due to the many advantages associated with the optical fiber medium. While optical fiber long distance trunks between central office switching points are well established, optical fiber paths have not yet been extended on a widespread basis to service subscriber locations. Central office locations provide a controlled environment for the optical transmission and reception apparatus associated with the cable. For instance, power supply levels are sufficient at the central office to provide needed heating and/or cooling to maintain temperature control of light emitting elements used in the optical communications path. While laser diodes are known to be useful for light beam communications over optical fibers, such laser diodes are not well suited for use at subscriber locations unless they are located in a house or building having a controlled temperature.
It is known that optical output power of a laser diode varies with temperature. The prior art has addressed this characteristic in several ways. One approach has been to control the environment in which the laser diode is operative. Heating and cooling devices have been employed to achieve a regulated temperature environment for the laser diode. One practical cooling mechanism for cooling a laser diode has been to employ junction devices implementing the Peltier effect at a junction of dissimilar materials. Joule heating has also been employed to provide heating to the laser diode.
One drawback of Peltier effect junctions and heating devices is that they are not efficient and typically require considerable operating power to achieve relatively modest heating and cooling effects. While laser diodes which are located at central office or head end interface units of an optical fiber communications network may be maintained within very close thermal tolerances, laser diodes of subscriber interface units remotely located from the office interface unit are typically subjected to much harsher thermal gradients, especially if located outdoors. Outdoor subscriber interface units are typically small weather-resistant enclosures which are located within about 200 feet of a subscriber's premises. Such units are typically exposed to the ambient environment. In some climates, the ambient environment may present a thermal gradient from -40 degrees Centigrade to +70 degrees Centigrade. Also, the power at each subscriber interface unit is very limited, and power is not available to provide Joule heating or Peltier cell cooling for the laser diode transmitter.
Optical fiber communications networks have typically been configured in a variety of architectures, including point-to-point, star, ring and bus. One advantage of a logical bus architecture is that it works particularly well within the conventional outside plant environment between the central office and subscriber premises.
With the logical bus architecture, time division multiplex techniques have been adopted to provide each subscriber interface unit with necessary access to the office interface unit at the central office or head end of the bus. Time division multiplex allots time slots to each of the subscriber interface units, and only one subscriber interface unit will send or receive data during its time slot in any given time frame. When laser diodes are operated only during a narrow time slot of an overall duty cycle, conventional power control techniques will not work since the laser diode is dark most of the time. Also, with subscriber interface units distributed along a logical bus, differing optical path losses require that a mechanism be provided to enable dynamic adjustment of the optical power level of each laser diode of the subscriber interface unit, so that those units most distant from the office interface unit put out the greatest optical power.
Thus, a hitherto unsolved need has arisen for a power control circuit for regulating optical power output of a laser diode operating in a time division multiplex mode and over a wide range thermal gradient.